This invention relates to a method of making a photolithographic mask. More particularly, it involves the use of an ion beam to selectively remove portions of a metallic layer on a transparent mask substrate in a desired pattern.
Photolithographic masks are widely used in semiconductor processing. One well known use for such masks is in selectively exposing resist on a semiconductor wafer. Briefly, these masks consist of a transparent glass substrate with a metallic coating on one surface. The metallic coating has openings therein in a desired pattern. Consequently, when the mask is placed over the semiconductor wafer, ultraviolet light, which used to expose the resist, passes only through the openings in the metallic coating on the mask. The remainder of the wafer is protected by the opaque metallic coating.
The pattern in the metallic coating for the masks has heretofore been made by chemical etching techniques. Briefly, the unpatterned metallic coating is covered with a layer of resist. Photoresists or electron resists have both been employed. Selected portions of the chosen resist are then exposed in the desired pattern. Using the photoresist, another mask is used to screen out undesired ultraviolet light. If the electron resist is used, then a low energy scanning electron beam can be used to selectively polymerize the electron resist. In either case, the unexposed resist must be washed away and the underlying metallic coating subsequently subjected to a chemical etchant bath. Using the resist as a protective screen, the unprotected portions of the underlying metallic coating are removed by the etchant to form the desired pattern. The resist is then removed to leave the completed mask.
In integrated circuit manufacturing, it is paramount that the mask pattern be well defined and preferably provide extremely fine line patterns. Unfortunately, the above mentioned techniques have their limitations. For example, with the electron beam exposure technique, the electron beam laterally scatters the electrons in the resist thereby causing a wider pattern than desired. Furthermore, the sensitivity of the electron beam positioning the ambient magnetic fields, the chromatic aberration effects which limit the current density, and the noticeable diffraction effects due to the non-negligible wave lengths of the electrons, all tend to inhibit accurate resolution of the mask pattern. It would be advantageous to provide a method of making a photolithographic mask in which the metallic coating on the transparent substrate could be selectively removed in the desired pattern without the use of the time consuming and inaccurate resist masking and chemical etching steps.